The invention relates generally to turbine buckets and, more particularly, to turbine buckets including an internal reinforcement lattice structure that serves to improve stiffness and vibration properties.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases. Energy is extracted from the gases in turbine stages for powering the compressor and performing external work.
Each turbine stage includes a stationary turbine nozzle having a row of nozzle vanes that discharge the combustion gases into a corresponding row of turbine rotor blades or buckets. Each blade includes an airfoil extending radially outwardly in span from an integral platform defining a radially inner flowpath boundary. The platform is integrally joined to a supporting dovetail having corresponding lobes mounted in a dovetail slot formed in the perimeter of a supporting rotor disk.
The turbine blades are typically hollow with internal cooling circuits therein specifically configured for cooling the different portions of the airfoil against the different heat loads from the combustion gases flowing thereover during operation.
The turbine airfoil includes a generally concave pressure side and circumferentially opposite, generally convex suction side, which extend radially in span from a root at the platform to a radially outer tip, and which extend axially in chord between opposite leading and trailing edges. The airfoil has the typical crescent radial profile or section that rapidly increases in thickness aft from the leading edge to the maximum width or hump region of the airfoil, which then gradually tapers and decreases in width to the relatively thin trailing edge of the airfoil.
In constructing a typical CMC (ceramic matrix composite) blade, plies are laid up onto the tooling surface from one side of the blade (either suction side or pressure side). As the layup process continues, the plies reach the midpoint or center of the blade airfoil. At this point, a mandrel is inserted into the tool, which produces the hollow cavity when the mandrel material is melted out. This mandrel contains ply wraps that produce the vertical “root to tip” thin walled features. The mandrel can be made from a variety of different materials, including, for example, pure tin, tin alloy, or an absorbable mandrel made from silicon/boron may be used. After the mandrel has been placed into the tool, the blade layup process continues through the blade.
In the current fabrication process, the blade has a tendency to uncamber or otherwise lose its curved airfoil shape. Additionally, existing buckets would benefit from improved stiffness and vibration properties.